Although integrin receptors have been shown to function as costimulatory molecules on mature thymocytes and T cells, it was not known whether these receptors could function as costimulatory molecules on immature thymocytes. Previous studies have shown that the expression of alpha 4 and alpha 5 integrins were significantly higher on immature, adult CD4- CD8- thymocytes than on either mature thymocytes or T cells, suggesting that these receptors are involved in early thymocyte development. We have shown that day 16 fetal thymocytes express levels of alpha 4 and alpha 5 equivalent to those of adult CD4- CD8- thymocytes. Immobilized fibronectin, a ligand for alpha 4 and alpha 5 integrins, was found to enhance the CD3-dependent proliferation of these fetal thymocytes. In the presence of IL-7, the magnitude of the proliferative response increased with time of incubation, resulting in a dramatic increase in the percentage of gamma delta thymocytes. The enhancement of proliferation by fibronectin was abrogated by soluble antibodies against alpha 4 and alpha 5, whereas immobilized mAb to alpha 4 and alpha 5 substituted for fibronectin in enhancing CD3- dependent proliferation, demonstrating that alpha 4 and alpha 5 integrins were responsible for the enhanced proliferation by fibronectin. Anti-alpha 4 mAb enhanced proliferation of fetal thymocytes by 100%, whereas anti-alpha 5 mAb and anti-CD28 mAb enhanced proliferation by 25%. Other costimulatory molecules, such as CD2, FcR gamma, and Thy-1, had no effect on the CD3 dependent proliferation of day 16 fetal thymocytes.In another study, several protein-nucleic acid complexes were observed when nuclear extracts from hepatoma cells were assayed for binding to the cAMP response element found in the phosphenolpyruvate carboxykinase-cytosolic (PEPCK-C) promoter. Although cAMP response element-binding protein and CCAAT/enhancer binding proteins alpha and beta have been identified as liver factors that bind this motif, an uncharacterized, slower migrating complex was also observed. We identified activating transcription factor-2 (ATF-2) as the factor in this complex and showed that ATF-2 stimulates expression from the PEPCK-C promoter. ATF-2 is a basic-leucine zipper transcription factor and a target for stress-activated protein kinases. We demonstrated that p38 beta mitogen-activated protein (MAP) kinase augments ATF-2 transactivation activity on the PEPCK-C promoter, which is consistent with the interpretation that PEPCK-C promoter activity is maintained under stress through a p38 MAP kinase dependent pathway. In this regard, we showed that treatment with sodium arsenite, a known activator of p38 MAP kinases, also stimulates expression from the PEPCK promoter. - Integrins, costimulatory molecules, cell adhesion, transcription factors, gene regulation, thymic differentiation, fibronectin